The present invention is generally directed to an electronic watch or clock and more particularly to a method and apparatus for performing temperature compensation of the temperature characteristics of a quartz crystal oscillator in which compensating data is stored in a read only memory ("ROM").
U.S. Pat. No. 3,719,838 uses data written directly into a programmable read only memory ("PROM") before compensating the temperature-frequency characteristic of a quartz crystal oscillator. Data corresponding to the temperature values is written directly into the PROM.
Japanese Patent Laid Open Publication No. 56-19482 discloses the use of temperature compensation data previously written into a Mask ROM wherein the address of the data is designated by the output conditions of a divider circuit. A given address is designated when the number of output pulses of a temperature sensing oscillator circuit reaches the number determined by a dividing ratio setting means.
Japanese Patent Laid Open Publication No. 58-223778 discloses a compensating circuit wherein the output of an A/D converter circuit is adjusted to produce a temperature value and wherein the temperature compensating data has previously been written into a Mask ROM. The data is called by the output of the temperature compensating circuit.
The temperature compensation method of U.S. Pat. No. 3,719,838 permits directly writing temperature compensation data, which corresponds to the temperature value, into a PROM, for compensating for the temperature characteristic of a quartz crystal oscillator. Thus, even though the secondary temperature coefficient and peak temperature of the quartz crystal oscillator may be varied, it is possible to adjust these parameters to correspond to the temperature value of each timepiece. This is an ideal temperature compensating method. However, the size of a nonvolatile memory circuit utilizing MNOS or FAMOS transistors in a PROM is three or four times as large as the size of MOS transistors used in a Mask ROM. For an electronic timepiece having an accuracy of approximately 5 seconds per year, a large memory capacity in the ROM is required. The size of the integrated circuit memory chip becomes extremely large and the circuit cannot be used in a wrist watch where space is limited.
In the other methods outlined above, the size of the chip is not an issue since a Mask ROM is used. However, while both methods provide a means for adjusting an offset temperature value, these methods do not provide a means for adjusting for the extent of the change in temperature; that is there is no adjustment made for slope. Thus, while it is possible to adjust for variation in the peak temperature of the quartz crystal oscillator circuit, it is not possible to adjust for variations in the secondary temperature coefficient. Therefore, the larger the departure from the peak temperature, the greater the error in pace. In order to obtain a high accuracy such as 5 seconds per year, special quartz crystal vibrators having sections with different secondary coefficients of temperature should be used. This results in very high manufacturing costs.